Rate-Sensitive Deformation Characteristics of Nanostructured Materials

Transcription

1 Rate-Sensitive Deformation Characteristics of Nanostructured Materials Koteswararao V. Rajulapati, Sreedevi Varam and K. Bhanu Sankara Rao School of Engineering Sciences and Technology University of Hyderabad, Hyderabad. 1

2 Driving force! Significant literature on mechanical properties of nano pure metals but not on two phase nanomaterials (both matrix and second phase at nano scale) Experimental investigations on deformation behavior of two-phase nanocrystalline materials and identifying the governing mechanics of plasticity in these materials as a function of grain size and second phase 2

3 Schematic representation of the variation of yield stress as a function of grain size. (Ref: K. S. Kumar, H. Van Swygenhoven, S. Suresh, Acta materialia, 51 (2003) 5743.)

4 Grain boundary sliding Grain rotation and coalescence Grain boundaries acting as sources/sinks for dislocations (Ref. M. A. Meyers, A. Mishra, D. J. Benson, Prog. Mat.Sci, 51 (2006) 427.)

5 Rate-sensitivity and activation volume The plastic deformation characteristics of nc-fcc metals are much more sensitive to the rate of loading than those of mc FCC metals. The SRS index is an order of magnitude higher for metals with nc microstructural features The activation volume of engineering metals and alloys is some two orders of magnitude smaller in nc metals than mc metals Ref: Q. Wei, S. Cheng, K. T. Ramesh, E. Ma, MSEA, 381 (2004) 71 5

6 Studies on rate sensitive deformation characteristics of electrodeposited nc-ni

7 Deformation behavior of nc Copper Ultra high strength, good ductility and dislocation based plasticity! Ref: K. M. Youssef, R. O. Scattergood, K. L. Murty, J. A. Horton, C. C. Koch, APL, 87 (2005)

8 Plastic deformation mechanisms that are responsible in coarse grained materials can not simply be extrapolated to the nano grained materials! Dislocation based plasticity or grain boundary mediated plasticity? SRS for bulk nanomaterials; in specific bulk two-phase nanocrystalline materials

9 Nano scale Two-phase alloys Ƭ = Gb/L L interparticle distance 40 nm 40 µm Nano scale Conventional scale Mechanics of plasticity depends on: Grain size of the matrix Second phase (size, distribution, amount etc.)

10 Effect of Pb on microhardness of nanocrystalline Al : Mechanical properties Minute additions of Pb result in precipitous decrease in hardness of nano Al! Average grain size of Al ~ 25 nm 10

11 Effect of Pb on microhardness of nanocrystalline Al : Microstructure Number fraction Mean grain size of Al = 28nm. Total 351 grains Grain size of Al(nm.) Pb particles 5 nm Nearly uniform grain size for Al in all the compositions; Ref: K. V. Rajulapati et al, Scripta Materialia, 55 (2006) 155. Broader distribution in Pb particle size 11

12 Pb segregates to the grain boundaries decreases the critical resolved shear stress required to nucleate the lattice defects from the nc Al grain boundary! Ref: K. V. Rajulapati et al, Scripta Materialia, 55 (2006) 155. S. Jang et al., Acta Materialia, 56 (2008)

13 Recent work XRD data of Al-Pb powders formation of two-phase structure; no solid solution formation average grain size of ~ 45 nm

14 2 cm X 1 cm (dia. X thickness) Bulk nanocrystalline samples are synthesized XRD data of spark plasma sintered Al-Pb samples After SPS@300 o C

15 Grain boundary controlled plasticity in FCC metals (based on MD simulations) It was suggested that the grain-boundary atoms as well as atoms upto 7-10 lattice parameters away from the grain boundary are heavily involved in plastic deformation. Deformation was mostly found to be taken up by atoms at and nearby grain boundaries It was further suggested that the material near grain boundaries is easier to deform, g.b.s are much softer compared to the grain interior and that the associated deformation mechanisms are rate sensitive However the specific deformation mechanisms have not yet been identified and more investigations are necessary Ref: V. Yamakov, D. Wolf, S. R. Phillpot, H. Gleiter, Acta Materialia, 2002 (50) 61 H. Van Swygenhoven, M. Spaczer, A. Caro, D. Farkas, Physical review B, 60 (1999) 22

16 Summary and Conclusions Mechanics of plasticity in two-phase systems at nano scale are complex; interplay between critical grain size Vs second phase Experimental evidence on SRS of two-phase bulk nanocrystalline materials SRS is an order of magnitude higher than conventional coarse grained materials The amount of nano-scale second phase enhances the SRS Contributions to higher SRS; smaller grain size -> more grain boundary area; second phase etc. Nanocrystalline materials could be reasonably ductile!

17 Acknowledgements Prof. K. Bhanu Sankara Rao IISc nano facility ARCI for SPS Prof. Vikram Jayaram and Dr. Samir Kamat DST for funding the work

18 Thank You!!!!!